1. Field of the Invention
The present invention relates to a workpiece table, provided for a machine tool, that is movable back and forth between two end positions and has a rotational drive which has a gear part joined to the workpiece table for rotation thereof, a toothed rack in engagement therewith, as well as a medium-actuated shift drive for displacement of the toothed rack, which thereby performs a linear stroke motion, delimited by stops, between two end locations corresponding to the end positions of the workpiece table.
2. Related Prior Art
A workpiece table of this kind, often also called a turntable, is known from practical use.
In many machine tools, the workpiece table is divided into a machining side and a loading side, such that a first workpiece can be machined on the machining side while a second workpiece, yet to be machined, is at the same time being clamped on the loading side. In this context, the machining side and loading side are often separated from one another by a spray shielding panel in order to protect the operator from flying chips and sprays of cutting fluid.
For tool changing, the workpiece table and the spindle head of the machine tool perform a movement relative to one another in which the machining side and loading side are interchanged.
One possibility for performing said workpiece change consists in using the turntable cited initially. Turntables of this kind are often used when the machine tool constructed therewith is intended to occupy as little space as possible.
For a workpiece change, the workpiece table is rotated alternately by the rotational drive through +/xe2x88x92180 degrees, the workpiece table being positioned in its end positions by means of, for example, serration splining. When the workpiece table is to be rotated, it is first lifted out of the serration splining before a toothed rack is then shifted from its one end location into the other, whereby it rotates a gear part and, by means thereof, the workpiece table into the respective other end position. After completion of this rotary movement, the workpiece table is lowered back down onto the serration splining.
The shift drive for displacing the toothed rack generally operates with compressed air, the two end surfaces of the toothed rack acting as thrust faces of a bidirectionally medium-actuated thrust piston. At the end of its respective linear stroke, the toothed rack arrives, with its end face upon which compressed air is not acting, against a stop, so that the linear stroke is delimited in both directions by stops.
The general desire in the mechanical engineering industry is to perform not only tool changes but also workpiece changes at shorter and shorter intervals, so that rotation of the workpiece table should occur as quickly as possible. A very rapid rotation of the workpiece table means, however, that the toothed rack arrives against the stops at a high velocity, resulting in hard impacts and shocks. Impacts and shocks of this kind are disadvantageous, however, in terms of workpiece clamping, since such shocks can result in loss of alignment in workpiece clamping fixtures, impairing the reproducibility of the machining operation. Another concern is that the workpiece table with the fixtures clamped thereon has a very large mass, so that with a corresponding rotational velocity, a relatively high rotational energy must be nullified in the end locations of the toothed rack. In the case of the known machine tool, shock absorbers, which extend outward away from the toothed rack at the stops and face the end faces of the toothed rack with their pistons, are provided for this purpose. Because of the high rotational energy that must be nullified, these shock absorbers protrude a great deal, so that overall they contribute to a large physical width for the known machine tool.
The general desire in the case of such machine tools is to make available machine tools with the minimum possible overall size, which allow the quickest possible workpiece change with no repercussions on the working accuracy of the machine tool. In order to meet this need, it is an object of the present invention to improve the workpiece table mentioned at the outset in such a way that while having a physically simple design, it allows rapid rotation with as little impact and jerking as possible; the external dimensions of the machine tool containing the new workpiece table are intended to be as small as possible.
In the case of the workpiece table mentioned at the outset, this object is achieved by the fact that a mechanism is provided which acts upon the toothed rack with a higher feed force at the beginning of the linear stroke movement than during the remainder of the linear stroke.
The object underlying the invention is thereby completely achieved. Specifically, the inventors of this application have recognized that by means of a xe2x80x9cstarting accelerationxe2x80x9d with a higher feed force, the inert mass of the workpiece table must first be overcome before the remaining rotary movement is performed with the usual feed force. This initially higher feed force thus ensures much faster rotation of the workpiece table between its two end positions, so that the workpiece change is thereby greatly accelerated. It is now entirely possible, by means of the new mechanism, to select the action upon the toothed rack in such a way that a high feed force is exerted initially, a usual feed force during the middle rotational range, and a lower feed force toward the end of the rotary movement, so that overall, the usual damping is sufficient to decelerate the rotating workpiece table in its end locations.
In an embodiment, it is preferred if the mechanism comprises an auxiliary shift drive which temporarily acts upon the toothed rack with an additional feed force at the beginning of the linear stroke movement.
This feature has the design-related advantage that the temporary additional feed force can be applied, for example, electrically or magnetically. A further advantage here is that said auxiliary shift drive can also be retrofitted, so that even existing machine tools can be subsequently refitted for more rapid rotation of the workpiece table.
The object underlying the invention is thus also achieved by means of an optionally retrofittable auxiliary shift drive for a rotational drive for a workpiece table of a machine tool which can be moved back and forth between two end positions, the rotational drive having a gear part joined to the workpiece table for rotation thereof, a toothed rack in engagement therewith, as well as a medium-actuated shift drive for displacement of the toothed rack, which thereby performs a linear stroke motion, delimited by stops, between two end locations corresponding to the end positions of the workpiece table, the auxiliary shift drive acting temporarily on the toothed rack with an additional feed force at the beginning of the linear stroke movement.
This auxiliary shift drive has the aforementioned advantage that it is retrofittable.
It is further preferred here if the auxiliary shift drive is medium-actuated, an auxiliary shift drive preferably being provided for each linear stroke direction of the toothed rack.
The advantage of this feature is that the same functional principle and actuation medium can be used for the auxiliary shift drive as for the original shift drive. This makes possible a generally simple physical design, since no additional energy sources need to be provided.
It is further preferred if the shift drive comprises thrust pistons at both end faces of the toothed rack acting respectively thereon, and if the auxiliary shift drive contains, at the two end faces, auxiliary thrust pistons acting respectively thereon, which preferably have a greater piston surface area than the thrust pistons and perform a shorter linear stroke than the latter.
The advantage here is that, so to speak, two thrust pistons one behind the other can be used, the auxiliary thrust piston, because of its greater piston surface area, exerting a greater feed force on the toothed rack for a given medium pressure than the standard thrust piston. Since the linear stroke of the auxiliary thrust piston is much shorter than the linear stroke of the toothed rack, the additional thrust force resulting from the auxiliary thrust piston acts only during an initial portion of the linear stroke of the toothed rack, thus providing, in an astonishingly simple manner, for an initially greater feed force, and then only the usual feed force, to act during the linear stroke movement of the toothed rack.
This simple design also allows for extremely uncomplicated retrofitting of existing machine tools with the auxiliary shift drive: the auxiliary thrust piston simply needs to be placed externally on the usual shift drive, the piston rod of the auxiliary thrust piston then acting, for example, directly on the thrust piston of the original shift drive. All that is then necessary is to connect the auxiliary shift drive to the medium line, which is already supplying the standard shift drive and providing it with compressive energy.
On the other hand, it is preferred if the toothed rack is configured as a bidirectionally medium-actuated thrust piston, on both of whose end faces an actuation medium, and a respective auxiliary thrust piston which has preferably a greater diameter than the toothed rack at its end faces and performs a shorter stroke than the toothed rack, act alternately.
This feature is advantageous in terms of design, since the toothed rack itself has at its end faces surfaces to be acted upon by pressure medium, on which the auxiliary thrust piston simultaneously acts during a short initial linear stroke and thereby exerts an additional feed force.
It is preferred in this context if each auxiliary thrust piston acts with its piston rod on the associated end face of the toothed rack, a stop for the auxiliary thrust piston being provided to delimit the linear stroke performed thereby.
This feature is also of design-related advantage, since the linear stroke movement of the auxiliary thrust piston is delimited in simple fashion. Since the mass of the auxiliary thrust piston and thus the energy to be nullified are negligibly small compared with conditions prevailing in the case of the workpiece table, the impact generated by the auxiliary thrust piston stop does not affect the machining accuracy of the new machine tool.
It is preferred in general if two damping mechanisms are provided, each acting between a stop for the toothed rack and the latter itself in order to decelerate the rotating workpiece table in its end positions.
This feature is known in the art; its purpose is to nullify the energy of motion of the workpiece table with as little impact as possible in order not to impair the operating accuracy of the machine tool.
It is preferred in this context if the damping mechanisms build up a regulated counterpressure for controlled deceleration of the moving toothed rack.
This feature, which is novel and inventive even when considered per se in the case of the workpiece table mentioned at the outset, ensures particularly gentle stoppage of the rotary motion of the workpiece table; these advantages are achieved even if no mechanism ensuring a temporarily higher feed force is provided.
With this feature, the thrust piston and/or the auxiliary thrust piston can now be used on the particular side of the toothed rack that is not being thrust upon, by building up in the corresponding pressure chambers a counterpressure which makes possible controlled deceleration of the toothed rack. This counterpressure can, for example, be elevated relatively quickly to a high value shortly before the end location of the toothed rack is reached, so that much of the energy of motion is nullified. The counterpressure is then released in controlled fashion so that it has approximately completely decayed when the toothed rack has reached its end location, preventing xe2x80x9cbouncingxe2x80x9d.
This feature now also allows faster rotation of the workpiece table, since the latter can be driven with a higher rotational velocity without resulting in greater impacts upon deceleration of the workpiece table, which would deleteriously influence the operating accuracy and reproducibility of the machine tools equipped therewith. It is possible, for example, to elevate the operating pressure for the usual shift drive so that a greater feed force acts during the entire linear stroke movement of the toothed rack and consequently also leads to a more rapid rotation of the workpiece table. The higher rotational energy associated therewith, which must be nullified when the end location is reached, is now absorbed by the regulated counterpressure.
Thus this new damping mechanism also achieves the object underlying the invention, because it ensures faster rotation of the new workpiece table and at the same time allows gentler stoppage of said workpiece table in its end positions.
The new damping mechanism can also, in and of itself, be retrofitted; in the simplest case, further compressed-air supply lines into the pressure chambers on both sides of the toothed rack simply need to be made available, provision then being made for regulation of the counterpressure via these two further compressed-air supply lines by means of, for example, a compressor. An already existing vent valve can, of course, also be refitted for this purpose.
The invention thus also concerns an optionally retrofittable damping mechanism for a rotational drive for a workpiece table of a machine tool which can be moved back and forth between two end positions, the rotational drive having a gear part joined to the workpiece table for rotation thereof, a toothed rack in engagement therewith, as well as a medium-actuated shift drive for displacement of the toothed rack, which thereby performs a linear stroke motion, delimited by stops, between two end locations corresponding to the end positions of the workpiece table, the damping mechanism building up a regulated counterpressure for controlled deceleration of the moving toothed rack.
The advantage of this damping mechanism is, as already mentioned, that it can also be retrofitted to existing machine tools, so that either gentler stoppage of the rotating workpiece table, or more rapid rotation of the workpiece table, is possible thereon, the new damping mechanism ensuring that the impacts occurring are no greater than in the case of the previous design of the rotational drive.
It is preferred in this context if the respective damping mechanism is configured on the shift drive and/or the auxiliary shift drive, and preferably generates, on the thrust side of the auxiliary thrust piston not presently being used to drive the toothed rack and/or on the associated end face of the toothed rack, a counterpressure which, during the return stroke of said auxiliary thrust piston effected by the toothed rack, decelerates the linear stroke movement of the toothed rack.
This feature is advantageous in design terms because it very elegantly combines the advantages of the auxiliary shift drive and those of the new damping mechanism. Initial experiments by the applicant of this application have indicated that with these features, a typical workpiece changing time can be reduced from 2.5 seconds to 1.5 seconds. Since a considerable effort must be made, in the case of the machine tools mentioned at the outset, to achieve time advantages on the order of tenths of a second, a time advantage of 1 second represents an enormous step forward, which reduces the workpiece-to-workpiece time for a correspondingly equipped machine tool to an unexpectedly high degree.
Additionally or alternatively, damping can also occur directly at the toothed rack. A separate vent valve is not even required for this; instead the compressed-air connection can serve as vent during the period in which it is not acting on the toothed rack; this requires simply a suitable external layout of the compressed-air hoses. Design changes to existing shift drives are therefore not necessary; they must simply be operated in the manner according to the invention in order to achieve the desired damping characteristics.
It is generally preferred in this context if the counterpressure has a regulated pressure profile; preferably each auxiliary thrust piston has on its thrust side, and/or the thrust piston has on its end faces, a respective pressure chamber to be vented by a vent valve, and the vent valve is a controlled valve, so that venting of the pressure chamber is enabled in controlled fashion, during the return stroke effected by the toothed rack, in such a way that a high counterpressure initially builds up for damping purposes, and is vented or limited when the toothed rack approaches its end location.
The advantage here is that the regulated counterpressure is provided for in a simply designed fashion. After displacement of the toothed rack into the one end location, the pressure chamber of the auxiliary thrust piston effecting said displacement is at the working pressure of the operating medium. When the toothed rack is then displaced back into the original starting location, it comes into contact, toward the end of its motion, with the piston rod of the auxiliary thrust piston, which consequently also performs a return stroke. During this return stroke the vent valve of the pressure chamber is first kept closed so that a high counterpressure gradually builds up. Said vent valve is then opened as a function of the location of the auxiliary thrust piston so that the pressure chamber is vented in controlled fashion until the toothed rack has reached its end location.
It is further preferred if the damping mechanism comprises a shock absorber which acts between the piston rod and stop.
The advantage here is that the shock absorber can nullify any unconsumed residual rotational energy of the workpiece table. This must be considered in the context that the rotational energy of the workpiece table depends, among other things, on the weight of the particular clamping fixtures being used, so that different rotational energies need to be nullified for differently equipped workpiece tables. Now, however, in order to adjust the controlled buildup of the counterpressure in as simple a fashion as possible, a shock absorber is additionally provided which compensates for the fluctuations in residual energy that were not absorbed by the counterpressure. In other words, the shock absorber is used whenever the rotational energy of the workpiece table was greater than could be absorbed and nullified by the counterpressure. As a result, however, the control system of the new machine tool is very simple.
In an embodiment, it is preferred if the shock absorber acts between the toothed rack and auxiliary thrust piston, the shock absorber preferably being arranged in the toothed rack and/or in the piston rod of the auxiliary thrust piston.
The advantage here is that the shock absorber no longer absolutely needs to be installed externally onto the auxiliary shift drive, so that the new machine tool can thereby be physically very narrow. The reason is that as compared with the prior art machine tool mentioned at the outset, the shock absorber must nullify much less energy, since much of the rotational energy is nullified by the counterpressure. The shock absorbers are therefore much smaller in this case than in the related art, so that they result in a much narrower machine tool even when installed externally. This is especially the case when they are housed in the toothed rack and/or in the piston rod of the auxiliary thrust piston. This is possible, as stated, because the shock absorbers in this case have much smaller dimensions than in the related art, where such an arrangement would not be possible.
The invention thus also concerns a machine tool which is equipped with a workpiece table in which the auxiliary shift drive and/or the damping mechanism are used.
The object underlying the invention is also completely achieved by this new machine tool, since because of the auxiliary shift drive ,the machine tool now makes possible a much faster tool change that is, however, also already possible due to the new damping mechanism. The new damping mechanism enables moreover the shock absorbers to be placed internally, so that the new machine tool is also physically much narrower.
It is further preferred in the case of the new damping mechanism if the regulated valve is a throttling valve that, when an opening pressure or a specific position of the piston is reached, preferably automatically vents the pressure chamber or operates with a constant throttling cross section.
The advantage here is first of all that with automatic opening, the regulated counterpressure can be, so to speak, automatically built up and then released again, or limited to a maximum; separate control devices therefor are not required, so that this feature has particular design-related advantages.
In the case of the constant throttling cross section, it is advantageous that a valve of very simple design can be used. Since said valve is, so to speak, always open, the result is a constant dissipation of discharge air, i.e. a continuous venting of the corresponding pressure chamber, the velocity of the pressure buildup and release being governed by the throttling cross section;. Because this throttling valve has no moving parts, it is not only economical but also highly reliable.
Further advantages are evident from the description and the appended drawings.
It is understood that the features mentioned above and those yet to be explained below can be used not only in the respective combinations indicated, but also in other combinations or in isolation, without leaving the context of the present invention.